init: Introduce Epoll::SetFirstCallback()

Prepare for optimizing Epoll::Wait() by moving the
ReapAnyOutstandingChildren() call into Epoll::Wait(). No functionality
is changed.

Bug: 213617178
Change-Id: I280ea0069ed29cf323e4177ec500b30b900f7c8d
Signed-off-by: Bart Van Assche <bvanassche@google.com>
diff --git a/init/init.cpp b/init/init.cpp
index ce668d7..cd0ee58 100644
--- a/init/init.cpp
+++ b/init/init.cpp
@@ -1061,6 +1061,11 @@
         PLOG(FATAL) << result.error();
     }
 
+    // We always reap children before responding to the other pending functions. This is to
+    // prevent a race where other daemons see that a service has exited and ask init to
+    // start it again via ctl.start before init has reaped it.
+    epoll.SetFirstCallback(ReapAnyOutstandingChildren);
+
     InstallSignalFdHandler(&epoll);
     InstallInitNotifier(&epoll);
     StartPropertyService(&property_fd);
@@ -1176,10 +1181,6 @@
         if (!pending_functions.ok()) {
             LOG(ERROR) << pending_functions.error();
         } else if (!pending_functions->empty()) {
-            // We always reap children before responding to the other pending functions. This is to
-            // prevent a race where other daemons see that a service has exited and ask init to
-            // start it again via ctl.start before init has reaped it.
-            ReapAnyOutstandingChildren();
             for (const auto& function : *pending_functions) {
                 (*function)();
             }